This invention relates to condenser efficiency in steam power plants for generating electricity. More specifically, it pertains to improving and integrating a pressure control device (PCD), which is the brain of a pressure control system1 (PCS), into the air removed system (ARS) of a shell and tube power plant condenser to improve the condenser's efficiency.
A condenser is an essential but also largely neglected component in a power plant for generating electricity. In a 1977 report (EPRI EF-422-8R) of the Electric Power Research Institute, Anson estimates: (1) that the loss of large fossil power plant availability directly attributable to condenser problems is 3.8%; and (2) condenser performance can significantly affect heat rate and generation capacity. In a 1988 report2, Piskorowski, et al, (EPRI-CS-5729) estimate that a 3377 pascal (one inch of mercury) increase in back pressure could result in a 2% reduction in generation capacity.
Air is a non-condensable gas and the presence of excess air in a condenser will significantly degrade the condenser's performance3. This is because the heat transfer surface is blanketed by a gas film which becomes highly resistance to heat transfer and lowers power plant efficiency. The amount of air inside a condenser, or air inventory, directly affects power plant condenser performance and is made up of two components, air in-leakage, and un-evacuated air pockets in a condenser tube bundle3. Un-evacuated air pockets in a condenser tube bundle is defined as the air in the tube bundle that cannot be removed by the current air removal system. Un-evacuated air pockets behave differently than the air moving around in a condenser in that they tend to be static. During steady state operation, the air in-leakage rate into a power plant condenser can be measured via a rotameter, however, the air inventory inside a condenser cannot be measured, but can be computed via a computer program such as the COMMIX-PPC4. The air in-leakage is due to the condenser operating below atmospheric pressure. Formation of air pockets is due to both poor design of tube bundle and inappropriate location of air off-take pipe. Both poor design of tube bundle and inappropriate location of air off-take pipe are common to most of condensers. A major concern in improving condenser performance, reliability and design is the efficient removal of air to keep air inventory inside a condenser to a minimum. It is to be noted that bio fouling is a maintenance problem and is not a condenser design problem.
To design an efficient and economic air removal system (ARS) for condensers, it is important to understand the major shortcomings of current ARS's. Since the majority of condensers are in-service condensers, special attention is given to them. The shortcomings are listed below.
The current design of air removal systems and condenser performance are de-coupled. The current design of air removal system is based on both the effective steam flow at each main exhaust opening and the total number of exhaust openings without requiring any knowledge of pressure, temperature, velocity and air concentration distributions inside the condenser.
The current air removal system is designed primarily to remove air in-leakage with the flow rate of vacuum pump as its boundary condition and is not designed to remove or reduce the size of air pockets in the condenser tube bundle. Since a condenser tube bundle has a complicated structure, it is difficult to measure or even to calculate both air concentration and pressure distribution in the tube bundles. As a result, the physical phenomenon of the formation and removal of air pockets in a tube bundle, which is critical to the condenser performance, is easily overlooked. The final shortcoming is that current air removal systems are passive systems. Their steady state operating pressure attains equilibrium by itself and cannot be changed.
In U.S. Pat. No. 6,128,9011, Sha teaches the use of a pressure control system (PCS) to enhance the performance of air removal systems in all existing power plant condensers. A pressure control system contains a pressure control device (PCD), which is a direct contact heat exchanger with sub-cooled liquid droplets to condense steam in a steam-air mixture from condenser exhaust, a variable speed pump, a chiller, flow measuring device, and temperature and pressure measuring devices. These components are connected to form a loop to circulate cold water to the PCD, where through a spray nozzle or orifice plate to form water droplets, a steam-air mixture from the condenser via an air off-take pipe (outside the condenser casing) is condensed in whole or in part. The operating pressure in the PCD is lowered as a function of the amount of steam condensed. The PCS adjusts the condensation rate in the PCD to yield an optimum minimum pressure. The condensation rate can be changed by adjusting either the flow rate or the temperature of the water from the chiller located upstream of the PCD. It is to be noted that the PCD pressure is essentially the same as at the inlet of a two-staged liquid-ring vacuum pump (TSLRVP) or the suction of a steam jet air ejector (SJAE). Thus an optimum pressure gradient between the condenser and the inlet of the vacuum pump or SJAE is created, which facilitates removal of air from the condenser. Also the PCS automatically changes the boundary condition at the inlet of the vacuum pump or the suction of the SJAE with its flow rate to the operating pressure of the PCD to greatly enhance air removal. It should be noted that if the steam flow rate entering a typical condenser is normalized to unity, the condenser exhaust flow rate is in the order of 0.02% of unity. The impact of condenser exhaust flow rate as boundary condition on removal of air is expected to be minimal as comparing to the pressure boundary condition.
The pressure control system lowers pressure at the PCD and therefore automatically reduces pressure at the air off-take pipe, to become the lowest, or close to the lowest pressure in the shell side of the condenser. Thus the efficiency of the existing air removal systems will be improved. If the air off-take pipe were located in a high pressure region in the shell side, air would move from the high pressure region to the lower pressure region and the efficiency of the air removal system would be lowered. With a PCS incorporated into an existing air removal system, it is expected not only to remove air in-leakage more efficiently, but also be capable of removing or reducing the size of air pockets in the condenser tube bundle. All current air removal systems for condensers are passive systems. Air removal systems with pressure control systems are no longer passive systems because the optimum gradient between the condenser and PCD is maintained at all times. With PCS incorporated into the existing air removal systems, all the shortcomings as outlined above are eliminated.